Effects of Drying Process with Different Temperature on the Nutritional Qualities of Walnut (Juglans regia L.)

Abstract

The chemical compositions, fatty acids, total flavonoids content (TFC), total polyphenols content (TPC), and minerals of walnuts were investigated after freeze drying (−55 °C) and heat drying (60 °C, 105 °C, 140 °C) process. The results showed that the contents of protein, soluble sugar, minerals (except for Mn), and carbohydrate were increased with the rise of temperature from −55 °C to 105 °C. Linoleic acid and oleic acid could be detected as the main fatty acids, with the highest contents were also observed at 105 °C (309.0 mg/g and 79.6 mg/g). However, the TFC and TPC were notably increased by 36.9% and 33.9% in kernel when the temperature was increased from −55 °C to 140 °C. In general, the freeze drying and heating drying had an inhibition effect and irritation effect on the nutritional qualities of walnuts, respectively. Therefore, heating treatment could significantly improve the nutritional qualities of walnut, and the recommended temperature for walnut drying was 105 °C.